Fix jvm target validation does not work for MPP JVM withJava target

Related KotlinCompile to JavaCompile wiring is happening on
KotlinJvmTarget initialization before 'withJava' configuration runs. At
that point linked JavaCompile task is null. Fix delays wiring to
execution phase when such task should be available.

^KT-55119 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/JvmTargetValidationTest.kt

@@ -321,7 +321,6 @@ class JvmTargetValidationTest : KGPBaseTest() {
     }
 
     @DisplayName("Validation should work correctly for KaptGenerateStubs task")
-    @GradleTestVersions
     @GradleTest
     internal fun kaptGenerateStubsValidateCorrect(gradleVersion: GradleVersion) {
         project(
@@ -387,6 +386,59 @@ class JvmTargetValidationTest : KGPBaseTest() {
         }
     }
 
+    @MppGradlePluginTests
+    @DisplayName("Validation should show error for MPP JVM target withJava")
+    @GradleTest
+    internal fun mppWithJavaFailValidation(gradleVersion: GradleVersion) {
+        project("kt-31468-multiple-jvm-targets-with-java", gradleVersion) {
+            gradleProperties.append(
+                """
+                kotlin.jvm.target.validation.mode = error
+                """.trimIndent()
+            )
+
+            subProject("lib").buildGradleKts.appendText(
+                """
+                |
+                |java {
+                |    targetCompatibility = JavaVersion.VERSION_17
+                |    sourceCompatibility = JavaVersion.VERSION_17
+                |}
+                """.trimMargin()
+            )
+
+            buildAndFail(":lib:compileKotlinJvmWithJava") {
+                assertOutputContains("'compileJava' task (current target is 17) and 'compileKotlinJvmWithJava' task" +
+                            " (current target is 1.8) jvm target compatibility should be set to the same Java version.")
+            }
+        }
+    }
+
+    @MppGradlePluginTests
+    @DisplayName("Validation should not run MPP JVM target without withJava")
+    @GradleTest
+    internal fun mppJvmNotFailValidation(gradleVersion: GradleVersion) {
+        project("kt-31468-multiple-jvm-targets-with-java", gradleVersion) {
+            gradleProperties.append(
+                """
+                kotlin.jvm.target.validation.mode = error
+                """.trimIndent()
+            )
+
+            subProject("lib").buildGradleKts.appendText(
+                """
+                |
+                |java {
+                |    targetCompatibility = JavaVersion.VERSION_17
+                |    sourceCompatibility = JavaVersion.VERSION_17
+                |}
+                """.trimMargin()
+            )
+
+            build(":lib:compileKotlinPlainJvm")
+        }
+    }
+
     private fun TestProject.setJavaCompilationCompatibility(
         target: JavaVersion
     ) {

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/targets/jvm/KotlinJvmCompilation.kt

@@ -6,6 +6,7 @@
 @file:Suppress("PackageDirectoryMismatch") // Old package for compatibility
 package org.jetbrains.kotlin.gradle.plugin.mpp
 
+import org.gradle.api.provider.Provider
 import org.gradle.api.tasks.TaskProvider
 import org.gradle.api.tasks.compile.JavaCompile
 import org.jetbrains.kotlin.gradle.dsl.*
@@ -23,7 +24,7 @@ open class KotlinJvmCompilation @Inject internal constructor(
 ) : AbstractKotlinCompilationToRunnableFiles<KotlinJvmOptions>(compilation),
     KotlinCompilationWithResources<KotlinJvmOptions> {
 
-    override val target: KotlinJvmTarget = compilation.target as KotlinJvmTarget
+    final override val target: KotlinJvmTarget = compilation.target as KotlinJvmTarget
 
     override val compilerOptions: HasCompilerOptions<KotlinJvmCompilerOptions> =
         compilation.compilerOptions.castCompilerOptionsType()
@@ -42,6 +43,10 @@ open class KotlinJvmCompilation @Inject internal constructor(
     override val compileTaskProvider: TaskProvider<out KotlinCompilationTask<KotlinJvmCompilerOptions>>
         get() = compilation.compileTaskProvider as TaskProvider<KotlinCompilationTask<KotlinJvmCompilerOptions>>
 
+    /**
+     * **Note**: requesting this too early (right after target creation and before any target configuration) may falsely return `null`
+     * value, but later target will be configured to run with Java enabled. If possible, please use [compileJavaTaskProviderSafe].
+     */
     val compileJavaTaskProvider: TaskProvider<out JavaCompile>?
         get() = if (target.withJavaEnabled) {
             val project = target.project
@@ -52,6 +57,19 @@ open class KotlinJvmCompilation @Inject internal constructor(
             project.tasks.withType(JavaCompile::class.java).named(javaSourceSet.compileJavaTaskName)
         } else null
 
+    /**
+     * Alternative to [compileJavaTaskProvider] to safely receive [JavaCompile] task provider  when [KotlinJvmTarget.withJavaEnabled]
+     * will be enabled after call to this method.
+     */
+    internal val compileJavaTaskProviderSafe: Provider<JavaCompile> = target.project.providers
+        .provider { if (target.withJavaEnabled) Unit else null }
+        .map {
+            project.javaSourceSets.getByName(compilationName).compileJavaTaskName
+        }
+        .flatMap {
+            project.tasks.named(it, JavaCompile::class.java)
+        }
+
     override val processResourcesTaskName: String
         get() = compilation.processResourcesTaskName ?: error("Missing 'processResourcesTaskName'")
 }

libraries/tools/kotlin-gradle-plugin/src/common/kotlin/org/jetbrains/kotlin/gradle/tasks/configuration/KotlinCompileConfig.kt

@@ -70,7 +70,7 @@ internal open class BaseKotlinCompileConfig<TASK : KotlinCompile> : AbstractKotl
 
     constructor(compilationInfo: KotlinCompilationInfo) : super(compilationInfo) {
         val javaTaskProvider = when (val compilation = compilationInfo.tcsOrNull?.compilation) {
-            is KotlinJvmCompilation -> compilation.compileJavaTaskProvider
+            is KotlinJvmCompilation -> compilation.compileJavaTaskProviderSafe
             is KotlinJvmAndroidCompilation -> compilation.compileJavaTaskProvider
             is KotlinWithJavaCompilation<*, *> -> compilation.compileJavaTaskProvider
             else -> null
@@ -78,9 +78,9 @@ internal open class BaseKotlinCompileConfig<TASK : KotlinCompile> : AbstractKotl
 
         configureTaskProvider { taskProvider ->
             taskProvider.configure { task ->
-                javaTaskProvider?.let {
-                    task.associatedJavaCompileTaskTargetCompatibility.value(javaTaskProvider.map { it.targetCompatibility })
-                    task.associatedJavaCompileTaskName.value(javaTaskProvider.name)
+                javaTaskProvider?.let { javaTask ->
+                    task.associatedJavaCompileTaskTargetCompatibility.value(javaTask.map { it.targetCompatibility })
+                    task.associatedJavaCompileTaskName.value(javaTask.map { it.name })
                 }
 
                 @Suppress("DEPRECATION")

tests/mute-common.csv

@@ -55,7 +55,6 @@ org.jetbrains.kotlin.idea.navigation.GotoWithMultipleLibrariesTest.testOneHasSou
 org.jetbrains.kotlin.idea.navigation.GotoWithMultipleLibrariesTest.testSeveralWithSource,,, FLAKY
 org.jetbrains.kotlin.tools.projectWizard.wizard.ProjectTemplateNewWizardProjectImportTestGenerated.GradleGroovy.testMultiplatformApplication,,,
 org.jetbrains.kotlin.tools.projectWizard.wizard.ProjectTemplateNewWizardProjectImportTestGenerated.GradleKts.testMultiplatformApplication,,,
-org.jetbrains.kotlin.gradle.NewMultiplatformIT.testJavaSupportInJvmTargets,,,
 org.jetbrains.kotlin.checkers.JavaAgainstKotlinBinariesCheckerTestGenerated.testJvmOverloadsFunctions,,, FLAKY
 org.jetbrains.kotlin.checkers.JavaAgainstKotlinBinariesCheckerTestGenerated.testReturnInnerClasses,,, FLAKY
 org.jetbrains.kotlin.checkers.JavaAgainstKotlinBinariesCheckerTestGenerated.testThrowsOnGenericMethod,,, FLAKY